Farmer Ants Fertilize Their Gardens With Bacteria

Thanks to their vast underground fungus farms, leafcutter ants are one of Earth’s most successful species — and one secret of their agricultural success is bacteria, which the ants use like fertilizer.

By farming with microbes that pull nitrogen from the air, the ants thrive in nitrogen-poor rain forest soil. Researchers say their bug-harnessing tricks might point people toward better ways of turning plants to fuel, or boosting our own crop yields.

“The reason we’re able to produce such massive crops is by the massive fertilization of nitrogen in our fields,” said University of Wisconsin bacteriologist Cameron Currie, co-author of a paper published Thursday in Science. “Ants supplement their crops through symbiotic associations with bacteria.”

A star of rain forest documentaries, leafcutter ants are one of about 250 ant species that subsist on farmed fungus. Most of these species live in colonies of a few thousand individuals, with tiny garden plots.

Leafcutter colonies have millions of members, with leaf-fed farms yielding more than a ton of fungus every year. Some scientists estimate they account for a full four-fifths of all living, nonplant rain forest matter.

Fascinated by their success, researchers have studied leafcutter gardening, but something wasn’t adding up. Though Earth’s atmosphere is nitrogen-rich, animals get their nitrogen by eating plants, or eating animals that eat plants. But rain forest foliage is nitrogen poor, as are the soils colonized by the ants.

“Nitrogen is one of the elements that ultimately determines productivity,” said Currie. “The nitrogen balance in ants is way off, based on what’s predicted from their diet.”

Currie’s team investigated the mystery of where the ants were getting their extra nitrogen by raising leafcutter colonies in airtight boxes. The soil in the boxes contained normal nitrogen. But the nitrogen in the air was replaced with a nitrogen atom with a different number of neutrons, called an isotope. By measuring the levels of the isotope in fungus and ant bodies, the researchers could track whether nitrogen was coming from the soil or the air.

They found that the fungus was getting nitrogen from the air. They then studied bacteria growing on the fungus, and found microbes from a genus called Klebsiella, which pulls nitrogen from the air at rates comparable to microbes that live on the roots of some plants.

“It’s entirely possible that nitrogen-fixing bacteria played a critical role in the evolution of this very different group of ants, with their giant colonies and massive effects on the environment,” said Ted Schultz, a Smithsonian Institute entomologist who was not involved in the study. He and Currie both noted that leafcutters are uniquely complex among fungus-growing ants, but evolved just 10 million years ago, or 40 million years after other fungus growers.

“What humans do for nitrogen is mine it from other sources, and dump it on our crops,” said Schultz. But this leads to waste and pollution, “and the ants accomplish it through microbes. Who knows? Maybe humans could do something similar, and cultivate microbial communities in the soil around our crops.”

Currie is studying whether nitrogen-fixing bacteria help break down the ants’ leaf cuttings into a fungally-digestible form. If so, the bacteria may suggest better ways of turning plants into biofuels. “We need to discover new enzymes, new processes, to convert plant cell walls into simple sugars that can be converted into ethanol,” he said. “Ants have been converting plant biomass into energy for millions of years.”

Currie added that leafcutter ants are the subject of thousands of papers authored over the last century, “yet this critical aspect of their success was completely unknown.”

“This is a well-studied natural system, and we’re still learning who the players are,” he said. “What does that say about most of the natural world, where mutalisms and associations haven’t been studied?”